The induction coil through which a moving workpiece is heated by magnetic induction is typically configured as a longitudinal flux inductor or transverse flux inductor. The workpiece may be a continuous, electrically conductive strip that passes through the induction coil. A longitudinal flux inductor is generally described as a solenoidal coil that surrounds the strip. AC current flowing through the solenoidal coil produces a magnetic field that is parallel with the longitudinal axis of the strip in regions where the field penetrates the strip. The magnetic field induces eddy currents in the continuously moving strip that heat the strip. A longitudinal flux inductor is superior to a transverse flux inductor for uniform cross sectional heating of a strip. However, when a longitudinal flux inductor is used to inductively heat a thin strip, a high frequency power supply, with attendant cost penalty, is required. Further the solenoidal configuration of a conventional longitudinal flux inductor makes it impossible to laterally move the continuous strip out from within the coil as may be desired, for example, to replace the existing inductor with a new inductor. The continuous strip must be cut to accomplish a change in inductors.
A transverse flux inductor can be used to inductively heat a thin strip at lower frequencies than those used with a longitudinal flux inductor. A transverse flux inductor is generally described as a pair of coils wherein the strip moves in a plane positioned between the planes in which the pair of coils are located. AC current flowing through the coils produces a magnetic field between the pair of coils that penetrates the strip and inductively heats it. Field penetration is generally orthogonal to the surface of the strip. Consequently the induced eddy currents are circulated in a plane near the surface of the strip, but not throughout the width or thickness of the strip. An additional advantage of a transverse flux inductor over a longitudinal flux inductor is that its configuration allows for lateral removal of a continuous strip from between the pair of coils.
In some applications, uniform cross sectional heating of the strip is not desired since the edges will cool down faster than the interior of the strip. For example, in a galvannealing process, a continuously moving strip is dipped into a liquid coating bath. The liquid coating thermally bonds with the strip after exiting from the bath. Since the edges of the strip will cool faster than the central region of the strip, the degree of bonding at the edges may vary to produce an unsatisfactory grade of galvanized product. In such cases, the edges must be scrapped from the galvanized strip product. Various types of dedicated edge heaters have been used to compensate for edge heat losses in a strip. U.S. Pat. No. 5,156,683 discloses a dedicated induction edge heater. The edge heater is preferably of the channel type, and requires the use of a mechanical drive system to reposition the edge heater as the width of the strip changes. Also a continuous strip will laterally oscillate as it moves along the heating line, so the mechanical drive system must be used to adjust the position of the edge heaters to accommodate this lateral motion. Further, in order to allow lateral removal of a continuous strip, the mechanical drive system must move at least one of the edge heaters away from the plane of the strip.
U.S. Pat. No. 5,837,976 discloses a coil system that allows lateral movement of a continuous strip similar to that provided by a transverse flux inductor while providing the advantages of a longitudinal magnetic flux field. The disclosed coil system comprises upper and lower coil sections that, together, form a two-turn solenoidal coil. AC current flowing through the coil sections results in a longitudinal flux field while a gap between the vertical bars or shunts connecting the two coil sections in series permits lateral movement of the strip out from the coil system.
It is one object of the present invention to provide a means for inductively heating a continuously moving workpiece, such as a strip, that allows for controlled edge heating of the strip without movement of the induction heating coils for strips of varied widths, while allowing unrestricted lateral removal of the continuous strip from within the induction heating coil assembly.